Sometimes when a rabbit starts to develop health problems, the other rabbit can sense it or it can cause the rabbit that is in pain to act defensively, and it can cause a disruption to their bond. So a thorough vet check for both might be a good idea, including a thorough dental exam for both.
https://rabbit.org/vet-listings/
Also look at any changes that might have occurred when this started happening. Changes in food, in their environment, in your environment, noises, smells, things physically moved or new things added. All sorts of things can be a cause of behavior changes in rabbits.
The only other thing I can think to try would be a complete separation for a few weeks, then attempt to rebond. It may or may not work though. You could even end up with two rabbits that don't want to rebond.
But an underlying health issue would be my first guess. It's often the case when a long term bond is disrupted. In the meantime, you could try scatter feeding their food and see if that makes it possible for them to eat together. Sometimes having to search for their food is enough distraction to the other rabbit.